[Report published in Astronomy & Geophysics 41, 1.29-1.30 (February 2000)]
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by N F Arnold (University of Leicester)
Published in Astronomy & Geophysics 41, 1.29-1.30 (February 2000)
The annual one-day meeting of the MIST (Magnetosphere, Ionosphere and Solar-Terrestrial) community was held on 26 November 1999 at the Geological Society, Burlington House. The organizer, Andy Smith (British Antarctic Survey), managed once again to pour a quart into a pint pot with 31 presentations over a wide range of topics without the schedule feeling overly rushed. The morning session was chaired by David Neudegg (Leicester) and began with four talks that concerned the Earth's magnetospheric cusp region between open and closed field lines. A combined Interball/Polar satellite study was described by Ian Krauklis (Mullard Space Science Laboratory) during high solar wind and northward Interplanetary Magnetic Field (IMF) conditions. Whilst the former was observing a lobe reconnection, the magnetosphere had been distorted such that the latter found itself within the cusp region. Jonny Rae (Leicester) combined results from Polar with ground-based observations from the CUTLASS Finland radar. Observations of 2-10 minute pulsations in the cusp region as measured by the spacecraft particle detectors could be related to pulsed ionospheric flows at the time of flux transfer events. Tim Stubbs (Imperial College) carried out a statistical study of Polar observations of the Northern Hemisphere cusp region. He found that the weak relationship between high solar wind speeds, the strength and direction of the vertical component of the IMF and the duration of expanded cusp conditions suggested that tail lobe reconnection was not the principal mechanism previously assumed. To complement the reporting of observational studies, Matt Taylor (IC) followed by describing a model of the interactions between low frequency waves and a cusp-like field geometry. Damping within the system suggested localized in situ source mechanisms. Additional processes were needed in the cusp to identify potential candidates.
Sean Oughton (University College London) presented a model of the Sun's outer atmosphere based on the driving of quasi-two-dimensional magnetohydrodynamic turbulence by low frequency Alfvén waves generated close to the photosphere. This mechanism may account for the much hotter corona compared with the photosphere. The next two talks dealt with the co-rotational interaction region within the solar wind. Will Wilkinson (Brighton) used the Rankine-Hugoniot relations that determine the physical parameters on both sides of MHD discontinuities to demonstrate that shocks with high Mach numbers can exhibit a well-defined rotation. Observations by the Ulysses spacecraft of these regions were reported by Murtaza Gulamali (IC). Fluctuations in the measured magnetic field were analysed using multi-resolution decomposition, revealing large structures with time scales of many hours and shorter scales characteristic of MHD turbulence. Silvia Dalla (IC) analyzed proton fluxes at around 5 AU using the Anisotropy Telescope on board Ulysses. A recurrent pattern emerged with a delay between events increasing from 122 to 155 days, possibly connected to the 154-day periodicity in the rate of solar flare occurrence. Matt Hopcroft (Warwick) presented 2-D simulations of super magnetosonic solar wind interactions with a small-scale comet. The bow shock structure was found to be sensitive to the flow Mach number and at high values generated a deflected velocity layer behind the shock.
The final three talks of the morning focussed on the opportunities presented by the total solar eclipse over Cornwall on 11th August 1999. Ruth Bamford (Rutherford Appleton Laboratory) reported nationwide efforts from the scientific community and the general public to gather a comprehensive and unique ionospheric record of the event. Large changes in radio signals were reported due to a significant reduction in the electron density. Ionosonde observations in Cornwall were described by Elizabeth Clark (Sheffield Hallam) to estimate the flux of ionizing radiation during the eclipse. She compared the recent eclipse with a similar event in July 1945 and noted significant differences that might be due to structural changes in solar activity. Finally, Tudor Jones (Leicester) discussed a number of experiments carried out to search for acoustic gravity waves within the thermosphere around the time of the eclipse. There was an unusual abundance of such disturbances on the day and waves with a direction of propagation consistent with a source region close to the path of totality were observed.
The first session of the afternoon was chaired by Malcolm Dunlop (IC). One of three consecutive speakers from Aberystwyth, Nick Mitchell described a new meteor radar system that has recently been deployed at Esrange in Sweden to monitor the dynamics of the upper mesosphere and lower thermosphere on a continuous basis. New interferometric techniques allow a vertical resolution of 2 km and a time resolution of an hour to be obtained. Defense Meteorological Satellite Programme data was combined with a network of high latitude radars by Adam Smith to investigate the progress of enhanced ionospheric density structures. He proposed that these were related to changes in the configuration of the IMF. Alison Canals analyzed European Incoherent Scatter radar interplanetary scintillation measurements of fast stream solar winds to identify signatures of the dissipation of low frequency Alfvén waves. Such a process has been proposed as a mechanism for accelerating the solar wind.
The next two talks took the audience to the Outer Planets. Rob Wilson (IC) analyzed data from twenty orbits of the Galileo spacecraft around Jupiter to confirm the suggestion from earlier flyby encounters that 10-20 minute waves existed in the magnetosphere. These are believed to be global resonance modes. Pioneer and Voyager data were also used by Stephan Espinosa (IC) to investigate the Saturnian magnetic field. A bipolar perturbation with a period close to the planet's rotation rate suggested a tilt between the rotation and dipole axes. Other observations appear to have ruled this possibility out, so other causes still need to be considered. Simulations of quasi-perpendicular shocks were presented by Robert Lowe (Queen Mary and Westfield College) to consider the mechanism causing the electron accelerations that have been widely observed. Of particular interest was the formation of ripples in the density and the magnetic field moving along the shock ramp. Eleri Pryse (Aberystwyth) presented results from a multi-instrument study using ground-based radars and satellite over-passes to explore the summer time ionosphere during southward enhancements of the IMF. In addition to the progressive equatorward motion of the plasma signature, the abrupt equatorward boundary of the electron temperature observed by EISCAT could be a signature of reconnection.
Kathryn McWilliams (Leicester) made use of the overlapping CUTLASS HF radar beams to generate the first multi-dimensional measurements of the convection pattern in the ionospheric footprint of a flux transfer event. The motion of this newly connected flux tube was compared with the ionospheric convection velocity. The relative strengths and weaknesses of a range of ionospheric sounding techniques were assessed by Natalia Nastasyina (Brighton) given the inherent uncertainties in both equipment and propagation media. She demonstrated from retrieval theory that the errors for the 'ordinary' polarization mode of radio waves increases from the magnetic pole to the equator, whilst the opposite is true for the 'extra-ordinary' mode.
The third and final session of the day was chaired by Peter Cargill (IC). Geomagnetic storms were a topic of conversation back in December 1128 AD according to David Willis (Warwick). From a number of old manuscripts, six distinct events, though intermittent, coincided with the synodic solar-rotation period. The timing of the storm suggested the phase of the solar cycle was near a maximum, whereas such events are associated with the declining phase during the current epoch. Will Wykes (Warwick) solved the complete set of the relativistic Lorentz equations numerically for the interactions of high speed electrons and whistler mode waves to show that the resulting chaotic interactions provide a mechanism for enhanced phase space diffusion. Thus electrons trapped in the radiation belt have an effective means of entering the loss cone and precipitating into the upper atmosphere. Two presentations from Leicester University considered the response of the ionosphere to the Interplanetary Magnetic Field. Firstly, Tim Yeoman described simultaneous interhemispheric radar observations of nightside ionospheric electric fields. The inclined phase front of the IMF at that time resulted in some asymmetries in the atmospheric response during a substorm growth phase and a substorm psuedobreakup interval. Secondly, Stan Cowley described how distortions of the magnetospheric field induced by the azimuthal component of the IMF can modify the ionospheric flow in such a way as to account for the measurements made by the EISCAT radar.
Matthew Denton (Sheffield) made use of the Sheffield University Plasmasphere Ionosphere Model to interpret observations from a DMSP satellite. Of particular interest was the question of whether readjustment of the neutral wind circulation or refilling of flux tubes was the dominant recovery mechanism following a geomagnetic storm. John Hargreaves (Lancaster) used the EISCAT VHF radar and the Kilpisjarvi imaging riometer to investigate a night-time precipitation event affecting the D-region of the ionosphere. A wave-like variation with a period of 25 minutes was almost certainly due to a pulsation in the particle fluxes, probably associated with high energy electron precipitation. A preliminary study of the magnetospheric data obtained from the Earth flyby of Cassini on its way to Saturn was given by Andrew Coates (MSSL). In addition to observing many of the expected boundaries and regions of the magnetosphere, there were a number of strange features including a narrow electron beam that required further investigation.
Roger Iles (MSSL) used data from two Defence Evaluation and Research Agency satellites to investigate the acceleration of relativistic electrons in the radiation belts. The acceleration process appeared to be related to solar wind speed, but less so to the intensity of the ring current. Two new experiments were introduced to the community to round off the meeting. Andy Lawrence (BAS) described a new auroral imaging system to be located at Halley, Antarctica early next year in order to investigate atmospheric gravity waves in the upper mesosphere and lower thermosphere region. In addition to providing a unique Southern Hemisphere dataset, it would allow inter-comparison with Northern Hemisphere observations. The first ever ground-based observations of the sunlit dayside aurora were reported by David Rees (Hovemere Ltd.). Sunlight overwhelms traditional detectors, but sophisticated optics have provided a means to investigate the low and mid-latitude thermosphere and ionosphere as well as the polar aurora during summer.
The meeting concluded with thanks to all the contributors, chairmen and organizer Andy Smith.